Emissions sources produce harmful air contaminants such as particulate matter (PM) and oxides of nitrogen (NOx). The United States Environmental Protection Agency (EPA) and state and local agencies continue to tighten maximum emission limits. In order to meet increasingly stringent regulations, engine and boiler manufacturers and operators install exhaust treatment systems to remove emissions from the exhaust stream before release to the atmosphere. The devices within the exhaust treatment must typically be operated within a certain temperature range.
An example of a device commonly used to remove PM is an exhaust filter such as a Diesel particulate filter (DPF). A DPF traps particles entrained in exhaust gases. Hydrocarbons constitute much of PM, which can be burned-off or “regenerated”. The typical DPF operating temperature range is between 540° C. to 650° C. during filter regeneration. If the exhaust gas temperature is too low, PM can build up and the DPF can clog. If a DPF runs too cold for too long, the filter can fail, cause a system failure, or the DPF can catch fire.
An example of a device commonly used to remove nitrous oxides (NOx) is a selective catalytic reduction (SCR) system. An SCR converts NOx into less harmful emissions, such as nitrogen and water. SCR systems may comprise a catalyst that facilitates a chemical reaction between the NOx and a reductant. The reductant may be added to the gas stream and is absorbed onto the catalyst before it reacts with the NOx in the exhaust gas stream passing through the SCR system. However, for this reaction to properly take place, the exhaust gas temperature must typically be between 200° C. and 315° C.
Thus, there is a temperature incompatibility problem which has not been adequately solved. As stated above, the typical temperature range for DPF's is between 540° C. to 650° C. (during regeneration) and the typical normal operating temperature range for SCR's is between 200° C. and 315° C. This is problem because DPF's and SCR's are typically positioned in series with each other in the exhaust gas stream.
Various methods to heat the exhaust gas for proper DPF regeneration have been tried. Some typical examples disclose an oxidation catalyst, such as a Diesel oxidation catalyst (DOC), upstream of the DPF. The DOC oxidizes CO to CO2, and/or NO to NO2 in an exothermic reaction that releases heat into the exhaust gas and increases the temperature of the exhaust gas. Other examples inject hydrocarbon (HC) fuel upstream of the DOC which is oxidized by the DOC in an exothermic reaction that raises the temperature of the exhaust gas prior to entering the DPF. Still others employ electric heaters upstream of the DPF. Still others heat the DPF substrate directly by passing electrical current through conductive DPF filter element(s). Still others employ fuel burner(s) upstream of the DPF.
In the case where the SCR is located downstream of the DPF, and when the DPF is periodically heated to regeneration temperatures, the entire exhaust gas stream is heated. The disadvantage of this configuration is that it exposes the SCR to excessive temperature when the DPF is regenerated.
In response to this disadvantage, a workaround arrangement was tried in which the SCR is located upstream of the DPF and a heat source inserted between them. This arrangement allows the SCR to operate at a lower temperature and then the heat source is used to raise the temperature of the exhaust gas before entering the DPF. A disadvantage of this workaround arrangement is that the SCR is exposed to the particulate matter (PM) in the exhaust gas stream, which coats the SCR catalyst, which can soon severely reduce the SCR's effectiveness.
Some have tried to cool the exhaust gas stream ahead of the SCR by various means so that the SCR catalyst is not exposed to excessively high temperatures. This is sometimes accomplished using engine coolant. The disadvantage of cooling methods is that they add cost, complication, waste energy, and possibly consume coolant through the conversion of liquid coolant into vapor.
A further disadvantage of the above cooling methods is that they only work when the exhaust gas temperature is sufficiently high already as it comes from the emissions source. This may work in some applications. However, in other applications, such as a remote emissions treatment system, the exhaust gas is too cool and therefore must be heated prior to the DPF. It is therefore a disadvantage to waste of energy to immediately cool the exhaust gas after it has just been heated.
Others have tried passive thermal regulation. This can be a disadvantage because SCR catalysts work best within a narrow temperature range. When a system has no means of thermal regulation, then temperatures can vary significantly. For example, temperatures may be lower than optimum when the engine is at idle. Conversely, temperatures may be higher than optimum when the engine is at full load.
Thus, there have been many attempts to solve the issues regarding the exhaust gas temperature incompatibility in systems that use DPF's and SCR's. Thus, there remains a need for efficient temperature management for these systems.